The field is internal combustion engines. Particularly, the field includes opposed-piston engines. In more particular applications, the field relates to a ported cylinder equipped with opposed pistons in which the bore and/or piston surfaces are constructed so as to promote lubrication of the bore/piston surface interfaces. Such constructions for a ported cylinder include the provision of an oil-retaining surface texture in an interface between opposed pistons disposed in the cylinder and the cylinder's bore. The oil-retaining texture includes one or more patterns of separate recesses that extend in a longitudinal direction of the cylinder, aligned with bridges of at least one of the cylinder's ports.
A “ported” internal combustion engine is an internal combustion engine having at least one cylinder with one or more ports through its side wall for the passage of gasses into and/or out of the bore of the cylinder. Relatedly, such .a cylinder is a “ported cylinder.”
When compared with four-stroke engines, two-stroke, opposed-piston engines have acknowledged advantages of specific output, power density, and power-to-weight ratio. For these and other reasons, after almost a century of limited use, increasing attention is being given to the utilization of opposed-piston engines in a wide variety of modern transportation applications.
A representative opposed-piston engine is illustrated in FIGS. 1 and 2. The opposed-piston engine includes one or more cylinders 10, each with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Each of one or more fuel injector nozzles 17 is located in a respective injector drilling that opens through the side of the cylinder, at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred as the “exhaust” piston because of its proximity to the exhaust port 14; and, the end of the cylinder wherein the exhaust port is formed is referred to as the “exhaust end”. Similarly, the piston 22 is referred as the “intake” piston because of its proximity to the intake port 16, and the corresponding end of the cylinder is the “intake end”. One or more rings 23 are mounted in circumferential grooves formed in each of the pistons 20, 22 near the piston's crown. When used herein, the term “ring” denotes a conventional piston ring and/or an annular, low-tension compression seal.
The exhaust and intake ports 14 and 16 of the cylinder 10 seen in FIG. 1 are similarly constructed. Consequently, although only the intake port construction is visible in the figure, the following explanation pertains to the exhaust port as well. As per FIG. 1, the intake port 16 includes at least one sequence of openings 28 through the sidewall and in a peripheral direction of a cylinder 10 near the intake end of the cylinder. For example, the openings 28 extend in a circumferential direction. The openings 28 are separated by bridges 29 (sometimes called “bars”). Relatedly, the term “port” in the description to follow refers to an alternating series of openings and bridges peripherally spaced around the cylinder near one of its ends. In some descriptions the openings themselves are called ports; however, the construction of one or more peripheral sequences of such “ports” is no different than the port constructions shown in the figures to be discussed.
Operation of an opposed-piston engine with one or more cylinders 10 is well understood. With reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) positions where they are at their closest positions relative to one another in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions in which they are furthest apart from each other. In an aspect of opposed-piston engine construction, the exhaust port 14 opens as the exhaust piston 20 moves toward BDC while the intake port 16 is still closed so that exhaust gasses produced by combustion start to flow out of the exhaust port 14. As the pistons continue moving away from each other, the intake port 16 opens while the exhaust port 14 is still open and a charge of pressurized air (“charge air”), with or without recirculated exhaust gas, is forced into the cylinder 10. The charge air entering the cylinder drives exhaust gasses produced by combustion out of the exhaust port 14.
As per FIG. 1, presuming the phase offset mentioned above, the exhaust port 14 closes first, after the pistons reverse direction and begin moving toward TDC. The intake port 16 then closes and the charge air in the cylinder is compressed between the end surfaces 20e and 22e. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 (typically, but not necessarily, diesel) is injected through nozzles 17 directly into the charge air in the bore 12, between the end surfaces 20e, 22e of the pistons. The mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 are near their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
In order to increase the mechanical effectiveness and durability of an opposed-piston engine, it is desirable to reduce energy loss and wear caused by friction between the cylinder bore and the opposed pistons disposed for sliding movement therein. In the opposed-piston context illustrated in FIGS. 1 and 2, there are three areas in which friction between the bore and the piston rings is most severe: 1) top reversal zones where the pistons reach TDC, 2) bottom reversal zones where the pistons reach BDC, and 3) the port bridges. The reversal zones are those annular sectors of the cylinder bore surface near where the pistons change direction and the reciprocating motion of the rings' sliding velocity is at zero.
When the sliding velocity of the piston rings is low enough, (as when approaching reversal zones), the hydrodynamic pressure of the oil film that keeps the rings and bores separated from each other diminishes. At that point the pressure difference between the inside and peripheral surfaces of the rings due to pressurized gases acting upon the inside face of the rings, the rings' tension, forced radial vibration forces, resonant radial vibration forces, and gravity force or any combination of such forces can induce asperities (roughness of the surfaces) of the rings and the cylinder bores to come into contact. When this happens, friction increases substantially and localized temperatures of the bore surfaces increase. This can result in the material at these locations failing if the strength of the bore's running surface material at a given temperature is exceeded.
Friction during these rough surface contacts is much higher than under conditions of pure hydrodynamic lubrication, (when, by definition, the asperities are not touching). Friction in the reversal zones typically contributes more than half of the total friction, power consumed by the pistons ring groups in spite of the low sliding speeds at these reversal zones. Reducing friction at these reversal zones has a large beneficial effect on overall friction of the ring system, as has been clearly demonstrated and documented in numerous technical papers, (i.e. “The Friction Force During Stick-slip With Velocity Reversal”, WEAR, vol. 216, Issue 2, 1 Apr. 1998, 138-149).
Very complex stresses occur during transit of the piston rings across the cylinder port bridges. Reduction of the bore surface area concentrates ring-loading pressure on the interface between the bridges and the ring surface portions that contact the bridges. The surface portions of the rings that pass over the port openings bulge and encounter the edges of the bore surface through which the openings are formed. These and other stresses produce high levels of friction as the rings pass over the ports.
To avoid failure modes and reduce overall friction for a given combination of bore running surface materials and ring running surface materials, asperity contact must be minimized, the coefficient of friction, and the temperature, must be reduced. One strategy to achieve these goals is to ensure that an adequate volume of oil resides in high-friction areas. The balance between pressure forces, viscous forces, oil cavitations, and surface tension forces supplies a net hydrostatic pressure that both reduces asperity contact and reduces friction.
The usual compromise with maintaining a layer of oil on the cylinder bore is that to some extent the oil will evaporate or will be mechanically depleted when exposed to the cylinder gases. This oil is lost either by being consumed in combustion or by being expelled as unburned, or partially burned, hydrocarbon in the exhaust stream, both of which result in undesirable consequences. The evaporation is aggravated as the vapor pressure of each of the oil's constituent fractions increases exponentially with temperature. Therefore, a significant amount of oil lost due to evaporation occurs in the top reversal zone. Mechanical depletion is aggravated when the thickness of the oil film becomes large enough that shearing forces from the sliding solid surfaces of the rings transport it either into the combustion chamber above the top ring or else into the exhaust port past the bottom ring. If oil is transported into the intake port, it may or may not be lost to the combustion chamber depending upon the gas flow conditions. Consequently, considerable attention has been given to the problem of maintaining a distribution of oil in the bore/piston interface, especially in zones of high friction.
One approach for retaining oil in the bore/piston interface is a cylinder bore construction including a surface texture composed of a plurality of indentations formed in the surface of the bore, particularly in the reversal zones. Lubricant retained in the indentations maintains the hydrodynamic film in those zones. For example, U.S. Pat. No. 7,104,240 describes a surface texture composed of a pattern of indentations formed in the bore of a cylinder liner in which a single piston slides on the bore surface between TDC and BDC areas that are located near the ends of the cylinder. In the pattern, the density of indentations varies in a longitudinal direction of the liner such that the density is greater at the longitudinal ends of the liner than in the middle. The density pattern spirals around the bore surface with a pitch that varies from end to end of the liner, in which the pitch is greater in the mid-portion of the liner than at the ends. Consequently, indentations are distributed circumferentially around the circumference of the bore surface, from one end to the other of the liner.
However, the longitudinal density variation of the spiral pattern of indentations in the liner bore for a single piston is unsuitable for the bore of an opposed-piston cylinder for at least two reasons. First, there are four reversal zones for the opposed pistons in the bore of an opposed-piston cylinder, with one BDC reversal zone at each end and two TDC reversal zones near the middle of the bore. Second, a continuous circumferential distribution of lubricant-retaining indentations in a ported cylinder would result in transport of lubricant past the port openings.